1. Technical Field
The present invention relates to a magnetostrictive torque sensor which has improved accuracy in detecting a torque on the basis of a change in magnetic characteristics resulting from a magnetostriction and a downsized or smaller electric power steering apparatus including the same.
2. Description of the Related Art
As a non-contact torque sensor, a magnetostrictive torque sensor, which detects a torque on the basis of a change in magnetic characteristics resulting from a magnetostriction, is known and used as a torque sensor for detecting a steering torque of an electric power steering apparatus (see, e.g., JP-A-2006-064445).
As shown in FIG. 4, a known magnetostrictive torque sensor 90, which is disclosed in JP-A-2006-064445, includes two magnetostrictive films 91, 92 provided on a rotary shaft 99. The magnetic anisotropies of the magnetostrictive films 91, 92 are different from each other. The magnetostrictive torque sensor 90 further includes a pair of detection coils 93, 94 arranged to face the magnetostrictive film 91 and to be separate from each other in an axial direction of the rotary shaft 99. First and second detection coils 95, 96 are arranged to face the magnetostrictive film 92 and to be separate from each other in the axial direction. As for the principle of the magnetostrictive torque sensor 90, when a rotational torque is applied to the rotary shaft 99, the magnetic permeability of the magnetostrictive films 91, 92 changes, wherein the inductance of each detection coil 93 to 96 also changes. The torque is detected based on a detected inductance change of each of coils 93 to 96.
For example, a torque detection output is calculated based on a difference between a detection output of the detection coil 93 and a detection output of the detection coil 96, or a torque detection output is calculated based on a difference between a detection output of the detection coil 94 and a detection output of the detection coil 95.
As for the manner in which the coils are arranged in a non-contact torque sensor, there is known an arrangement in which an exciting coil and a detecting coil are disposed at a same position with respect to an axial direction of a rotary shaft. There is also known an arrangement wherein the exciting coil is disposed outside the detecting coil in a radial direction (see, e.g., JP-A-11-059450).
Meanwhile, in the known magnetostrictive torque sensor of FIG. 4, a length of a magnetostrictive film in an axial direction is set according to a positional relationship of the detection coil facing the magnetostrictive film. However, a relative position between the magnetostrictive film and the detection coil in the axial direction varies due to a variation in the length of the detection coils in the axial direction or a variation in the position in which the detection coils are attached inside a gear housing. Therefore, in order to allow for a variation of the relative position between the magnetostrictive film and the detection coil in the axial direction, it is effective to set the axial length of the magnetostrictive film to be sufficiently longer than an axial length of the detection coil.
For example, in the case of the magnetostrictive torque sensor 90 shown in FIG. 4, an axial length of the magnetostrictive film 91 is set to be longer than a total axial length measured from an upper end of the detection coil 93 to a lower end of the detection coil 94. Similarly, an axial length of the magnetostrictive film 92 is set to be longer than a total axial length measured from an upper end of the detection coil 95 to a lower end of the detection coil 96.
However, when the axial lengths of the magnetostrictive films 91, 92 are set as described above, the total axial length measured from the upper end of the magnetostrictive film 91 to the lower end of the magnetostrictive film 92 becomes rather long since the detection coils 93 to 96 are arranged in the axial direction. Accordingly, the total length of the rotary shaft 99 becomes too long, such that a size of the magnetostrictive torque sensor 90 becomes rather large, thereby deteriorating mounting performance to a vehicle or an apparatus.
Furthermore, when the sensor 90 is used in a condition where a bending moment is applied to the rotary shaft 99, a strain due to the bending moment causes an error of the detected torsional torque.
In the magnetostrictive torque sensor 90 shown in FIG. 4, wherein the four detection coils 93 to 96 are arranged in the axial direction, the rotary shaft 99 is supported at one end thereof by a bearing 98. When a force P acts on a tip of the rotary shaft 99 in a direction that is orthogonal relative to the axial direction, the lengths L1, L2, L3, and L4 from the respective detection coils 93, 94, 95, 96 to the point of action are different from one another. Therefore, the bending moments at the respective positions of the detection coils 93 to 96 become P·L1, P·L2, P·L3, and P·L4, respectively, which are different from one another, wherein the strains in the respective detection coils 93, 94, 95, 96 are also different from one another. The different bending moments and corresponding strains generate an error in the detected torsional torque. In particular, when the axial distances between the respective detection coils are relatively long, the differences between the bending moments increase, wherein an error in the detected or measured torque increases.
Additionally, when the magnetostrictive torque sensor 90 is used as a steering torque sensor to detect the steering torque of a vehicle, the steering torque sensor is installed in a steering shaft between a steering wheel and a steering gear box. When the vehicle is stopped after a period of high-speed driving, the steering box is heated by the heat from the engine wherein the steering shaft is also heated. In such a case, a temperature distribution in the steering shaft is not uniform, and the temperature in a lower portion of the steering shaft closer to the engine becomes higher than that of an upper portion of the steering shaft which is remote relative to the engine. Therefore, the temperature of the steering shaft where the lowermost detection coil 96 is arranged becomes high, and the temperature of the steering shaft where the uppermost detection coil 93 is arranged becomes relatively low.
The magnetostrictive films 91, 92 have a temperature characteristic wherein their magnetic permeability μ increases as temperature increases. Therefore, when a temperature gradient is generated in the steering shaft in the axial direction thereof, an impedance of a detection circuit changes so that a detection output varies, wherein detection accuracy of the sensor decreases. When the axial length between the magnetostrictive films 91, 92 increases, a temperature difference accordingly increases. In such a case, a variation in the detection output increases wherein the detection accuracy decreases.